Sugar drying method

ABSTRACT

In a process for drying droplets of sucrose-containing solution in a current of heated air and in the presence of 0.5 to 4.0 parts by weight of separately introduced recycled product solids per part by weight of solids in said solution the improvement which comprises controlling the average particle size of the recycled product solids at about 200 microns or less and dispersing the recycled fine-sized solids at the outer periphery of the drying zone as an enveloping solids-bearing atmosphere around the solution droplets. Apparatus is disclosed for carrying out the process including a spray drying chamber, a centrally located liquid atomizer, drying air inlet and outlet conduits, solids product collection means, means for deagglommerating product solids, and means for recycling product solids to the drying chamber and introducing them tangentially as an enveloping curtain of solids adjacent the walls of the drying chamber.

United States Patent Woodruff et a1.

[4 1 Dec. 19, 1972 [541 SUGAR DRYING METHOD [72] Inventors: Edward T.Woodrufi, Woodbine, Md.; Viggo S. Andersen, Hackettstown,N.J.

[73] Assignee: W. R. Grace & Co., New York,

[22] Filed: May3, 1971 [21] Appl. No.: 139,664

[52] US. Cl. ..l27/62, 99/199, 127/16, 127/58, 159/4 S [51] Int. Cl...C13f H02 [58] FieldofSearch ..127/58,61,62;159/4R,4S

[56] References Cited UNITED STATES PATENTS SUCROSE SYRUP PrimaryExaminerMorris O. Wolk Assistant Examiner--Sidney MarantzAtt0rneyWilliam W. McDowell, Jr. and Kenneth E. Prince {571 ABSTRACT Ina process for drying droplets of sucrose-containing solution in acurrent of heated air and in the presence of 0.5 to 4.0 parts by weightof separately introduced recycled product solids per part by weight ofsolids in said solution the improvement which comprises controlling theaverage particle size of the recycled product solids at about 200microns or less and dispersing the recycled fine-sized solids at theouter periphery of the drying zone as an enveloping solidsbearingatmosphere around the solution droplets. Apparatus is disclosed forcarrying out the process including a spray drying chamber, a centrallylocated liquid atomizer, drying air inlet and outlet conduits, solidsproduct'collection means, means for deagglommerating product solids, andmeans for recycling product solids to the drying chamber and introducingthem tangentially as'an enveloping curtain of solids adjacent the wallsof the drying chamber.

10 Claims, 2 Drawing Figures PATENTEDnEc 19 I972 SHEET 1 [IF 2 Fig.

mm mm INVENTORS Wood ruff n5m w mwOmonw Andersen ATTORN EY PATENTEn DEC19 I972 SHEET 2 [IF 2 Fig.

mnmrw mwOmoDw ATTORNEY SUGAR DRYING METHOD I This invention relates, toimprovements in the production of dried sucrose-containing products fromsucrose-containing solutions ln one particularembodiment it relates to aprocedure for ensuring continuous operation of a process for spraydrying sucrose-containing solutions in the presence of recycled productsolids; and at the same time eliminating the need for any postcrystallization treatment of product solids between recovery from thedryer outlet and recycle to the dryer solids feed inlet. I

In the conventional methods of producing crystalline sucrose, the virginsyrup, as extracted from sugar cane or sugar beets, is first decolorizedand deionized. The clarified syrup is then evaporated and fractionallycrystallized in a series of precisely controlled steps, carmelizationbeing reduced as much as possible by the use of vacuum to reduce theevaporation temperature. Fractional crystallization is essential since,during processing, inversion of the cane sugar takes place, usually tothe extent of about 15 percent by weight of the original sucrose contentby the time processing is completed.'This invert sugar, which issubstantially uncrystallizable, is discarded together with up to about20 percent of non-recoverable sucrose and finds its way into commerce asanimal feed or other low grade sugar products generally referred to asmolasses.

Other methods for producing dry crystalline sucrosecontaining productsproposed by the prior art include fluid energy milling (Reimers et al.U.S. Pat. No. 3,140,201) drying intimate blends of sugar solids andsugar solutions (Oikawa U.S. Pat. No. 3,271,194 and British Pat. No.1,099,723) and simultaneous centrifugal distribution and spray drying ofsugar solids and a sugar solution (Japanese Pat. publication No.20384/69). Agglomeration of pulverized sugar using a liquid sugar syrupbinder and drying of the agglomerated product is taught in Harding etal. U.S. Pat. No. 3,518,095. A similar agglomeration process isdisclosed in Gidlow et a l. U.S. Pat. No. 3,506,457. A process andapparatus for spray-drying liquid substances in the presence of smallamounts of dust-like particles (e.g., recycled fines) is taught byBishop U.S. Pat. No. 2,698,815. Molasses is one liquid product suggestedas suitably dried in this manner. Spray drying of high D.E. glucosesolutions in the presence of recycled solids is disclosed by Repsdorphet al. in U.S. Pat. No. 3,477,874 and corresponding British Pat. No.1,075,161. A process and apparatus for spray drying a foamed lactosesyrup is disclosed in Nava et al. U.S. Pat. No. 3,533,805. Process andapparatus for preparing a spray dried topping mix containing sucrose,some of which is added in the dry state to the dryer, is disclosed inNezbed U.S. Pat. No. 3,414,980.

Belgian Pat. No. 742,249 dated Jan. 30, 1970 discloses a process fordrying sucrose-containing solutions by dispersing sucrose particles in acurrent of heated air, separately dispersing the solution on the saidparticles, evaporating the water from the solution coated on theparticles and recovering the resulting dry product from the hot airstream. In a preferred embodiment of the disclosed process a portion ofthe dried product is recycled as the solids dispersed in the dryingzone. In the process of the Belgian patent the use of sucrose particles,e.g., recycled dry product, is necessary to the process of drying thesucrose-containing solution. Attempts to directly spray dry the solutionin the absence of such solid particles results in caramelization anddegradation of the sugars in the solution and irreversible adhesion andsticking of the freshly formed, incompletely crystallized solids to thewalls of the dryer.

The presence of recycle solids provides a practicable process for spraydrying sucrose-containing solutions. However, some problems still remainin attempting to practice this process on a commercial, high productionrate, continuous basis.,For example, in pilot plant runs of the processdescribed in the aforementioned Belgian Pat. No. 742,249, the productcoming from the dryer is typically a plastic-like mass of incompletelycrystallized material. in a short period of time (e.g., 5 to 10 minutes)crystallization of the product will be complete and a free flowingproduct results, comprising agglomerates of sucrose microcrystals withagglomerates ranging in size from about 1,000 microns (roughly the sizeof the individual crystals of commercial white household sugar) to aslow as about 150 microns and made. up of individual microspheres rangingin size from about 25 to about 150 microns. Average agglomerate particlesize typically ranges from about 200 to 600 microns (i.e., 50 weightpercent of the product will be above and 50 weight percent below a givenparticle size in the range of about 200 to 600 microns).

When the 200 to 600 micron product particles or agglomerates arerecycled to the drying step they tend to promote heavy accumulation ofthe plastic mass on the dryer walls. Portions of this mass flake off thewalls and discharge from the dryer in a random intermittent fashion.From time to time the size of the randomly discharged portion is solarge that it effectively blocks the dryer outlet, forcing a shut-downand interruption of continuous operation.

It is an object of, the present invention to resolve these prior artproblems. i

It is a specific object of the invention to provide a procedure forinhibiting or preventing large build-ups of uncrystallized plasticmaterials on the dryer walls, thus ensuring continuous operation for along period of time.

Still another object of the invention is the provision of a dryingprocedure and apparatus which eliminates the need for anypost-crystallization steps or equipment.

Other objects and the advantages of the invention will be evident fromthe following description thereof.

in accordance with the present invention, build-up of solids on thedryer walls during the drying of sucrosecontaining solutions in thepresence of recycled product solids is inhibited or prevented, andcrystalline free flowing product is obtained directly from the dryeroutlet (i.e., without the need for any post-crystallization externallyof the dryer) by maintaining the average particle size (i.e., the sizewhere 50 weight percent of the particles are large and 50 weight percentare smaller), of the recycled solid product at about 200 microns orless, preferably about 150 microns or less and most preferably betweenabout 50 microns and or microns and by dispersing the recycled finesizedproduct solids at the outer periphery of the drying zone as anenveloping solids-bearing atmosphere around the solution droplets. Inthis way wall accumulation in the dryer is negligible andpost-crystallization is eliminated. 1

- In a particular embodiment the invention is applied to the sucrosesolution spray drying process which has been described in theaforementioned Belgian Pat. No. 742,249. In general, the processinvolves atomization of the sucrose-containing solution to be dried intothe hot air flowing in the spray dryer while separately feedingsubstantial proportions of solid sucrose particles. Thesucrose-containing solution may contain from about 60 to about 90 weightpercent solids. Where appropriate or desirable, the solution ispre-heated, e.g., to a temperature of 50 to 100 C. to preclude prematurecrystallization and to aid in feeding and atomization. The solidsucrose, preferably recycledproduct, is separately and concurrently fedto the dryer in amounts sufficient to provide a weight ratio of from 0.5to 4.0, preferably about 1.5 to about 3 parts of solids, for each partby weight of, solids in the solution to be dried. Stated conversely, theweight ratio of solids in the solution to the separately introducedproduct solids ranges from about 0.25 to about 2, preferably from about0.33 to about 0.67. The drying temperature (i.e., the ambienttemperature in the dryer after substantially steady state conditionshave been established for continuous operation) will vary with otherprocess conditions and the particular sucrose-containing solution to bedried but typically ranges from about 50 to about 130 C., preferablyabout 80 to about 100 C. Temperature can be controlled by controllingthe temperature or feed rate of the incoming sugar syrup or the recycledsolids or the temperature of the incoming hot air, or the averageresidence time in the dryer.

The particle size of the recycled solids is maintained in the desiredrange by size reduction prior to the recycle. This can be accomplishedby an intermediate grinding step, by appropriately designing the recyclesystem so that the product solids are deagglomerated by attrition orimpact in the course of being returned to the dryer solids feed inlet,or in any other suitable manner. In anyevent it is essential that theaverage particle size of the recycled product solids be reduced to notgreater than about 200 microns and preferably not greater than about 150microns prior to reintroduction into the dryer. For reasons of overallprocess economy, dusting losses and the like, it is generallyundesirable to reduce the average particle size of the recycle productlower than about 25 microns, particularly since no substantial furtherimprovement in results will be observed. In the presently preferredembodiment of the present invention the product solids aredeagglomerated in the solids recycle system to an average particle sizewithin the range of from about 50 to about 100 or 15.0 microns.

Size reduction of recycle product solids particles as an improvement,standing alone, is the subject of commonly owned, concurrently filedU.S. Application Ser. No. 139,852.

In accordance with the present application still further improvementsare achieved by recycling the reduced size product solids at the outerperiphery of the drying zone as an enveloping solids-bearing atmospherearound the atomized sucrose-containing solution to be dried. This notonly further inhibits accumulations on the dryer walls but results inthe further unexpected advantage of providing directly from the dryer adry, free-flowing product.

There are several ways of introducing the recycled product in accordancewith thepresent invention. In one embodiment, the recycled product isintroduced into the dryer by means of a centrally located centrifugalpowder atomizer and is carried by drying air currents to the dryerwalls. In another, presently preferred, embodiment the recycled productsolids are recycled in a current of conveying gas (e.g., in a pneumaticconveyor) and introduced tangentially along the dryer walls at aplurality (preferably from two to eight) of points spaced about thedryer periphery. It is not necessary and may not be desirable for theplural tangential inlets to be equally spaced about the dryer periphery.In any event the sucrose-containing solution is separately atomized inany suitable manner (e.g.,'centrifugally or in a single or multiplefluid sprayer) centrally of the drying zone and within the envelopingsolids-bearing atmosphere.

. The drawings schematically illustrate apparatus suitable forpracticing the present invention. In the drawings:

FIG. 1 is a schematic illustration of apparatus incorporating acentrifugal powder atomizer.

FIG. 2 is a schematic illustration of apparatus in which recycle solidsare tangentially introduced at the dryer walls.

Ambient air from supply fan 1 passes through line 2 into indirect heater3 to raise the air temperature from ambient conditions to proper spraydrying temperature. The heated air continues in line 4 and enters thespray dryer 24 via annulus 5.

The sucrose syrup to be spray dried is received in feed tank 6. Fromthere it is pumped via pump 7 through heat exchanger 8 and through line9 entering the dryer through atomizer head 10.

Spray dried product drops from the chamber of dryer 24 and passesthrough line 11 and into surge hopper 12. From there it is fed by avolumetric feeder 13 into a pneumatic conveying system. The pneumaticconveying system constitutes supply fan 15 and transfer line 14 whichconveys recycle product back to spray dryer 24. Recycle product alongwith its conveying air enters the dryer through centrifugal powderatomizer 16. From the atomizer 16 the powder particles are carried bythe drying air currents to the walls of the dryer where an evelopingsolids-bearing atmosphere,

' generally designated 17, is formed about the atomized sucrose solutionspray droplets, formed by atomizer head 10. In the course of recycle tothe dryer the product particles are passed through a grinder (not shown)or are otherwise suitably reduced to an average particle size of aboutmicrons or less prior to introduction into the dryer. Some or all of thesize reduction may be accomplished in the powder atomizer'l6. Productnot recycled overflows surge hopper 12 and is transferred to storage andpackaging via line 25 Dryer off gases pass from line 18 into cyclonecollector 19. Separated product fines pass from collector 19 throughline 20 where they enter line 14 and mix with product from feeder l3.Solids-free off gas from collector 19 passes through line 21 and isexhausted to the atmosphere via exhaust fan 22 and vent line 23.

In the apparatus of FIG. 2 ambient air from supply fan 31 passes throughline 32 into indirect heater 33 to raise the air temperature fromambient conditions to proper spray drying temperature. The heated aircontinues in line 34 and enters the spray dryer 55 via annulus 35.

The sucrose-syrup to be spray dried is received in feed tank 36. Fromthere it is pumped via pump 37 through heat exchanger 38 and throughline 39 and enters the dryer through centrifugal atomizer 40.

Spray dried crystaline free flowing product falls from drying chamber 55through line 41 into surge hopper 42. From there it is fed by avolumetric feeder 43 into a pneumatic conveying system via line 44. Thematerial passes through a combination pneumatic conveying and millingfan 45 which reduces the particle size of the product. The material isthen conveyed through line 46 and recycled to spray dryer 55.

Recycle product enters dryer chamber 55 through tangential inlets 47.Having been placed in the dryer in such a manner the recycle productflows along the dryer walls and forms a solids-bearing envelopingatmosphere generally designated 48 between the dryer walls and the syrupspray droplets from 40.

Product not recycled to drying chamber 55 overflows surge tank 42 andpasses through line 56 to product storage and packaging.

Exhaust gas from drying chamber 55 passes through line 49 to cyclonecollector 50. Product fines from 50 are fed into pneumatic recycle line44 through line 51. Solids-free off gas from collector 50 goes throughline 52 to exhaust fan 53 and discharge to the atmosphere via line 54.

Any number of sucrose-containing solutions may be dried in accordancewith the present invention; including, for example, refined sugarsyrups, raw sugar syrups, affination syrups, syrups resulting fromvarious strikes (e.g., the second, third or subsequent strikes) inconventional sugar crystallization processes, remelt syrups, ediblemolasses, and the like; or mixtures of the foregoing. The solutions areappropriately decolorized, where required, if a white product is desiredand are concentrated or diluted to the desired solids content prior todrying.

The invention will be further understood from the following illustrativeexamples:

EXAMPLE A (Comparative Example) A pilot plant run was conductedutilizing the process and apparatus described in Belgian Pat. No.742,249. The spray dryer apparatus had a 7% foot diameter with a 3% foothigh cylindrical section and a 60 conical bottom giving an overall dryercapacity of approximately 240 cubic feet. The liquid atomizer was a highvane centrifugal wheel rotating at 22,900 revolutions per minute.

The sucrose solution feed was afiination syrup from a commercial canesugar refinery. This syrup contained about 70.5 weight percent solids.It was preheated and fed to the dryer at a temperature of about 170 F.and at a rate of 16.2 gallons per hour, providing a syrup solids feedrate of about 128 pounds per hour.

Dried, solid sucrose product from an earlier run was used for theinitial recycle. This product had an average particle size of about 300microns. Thereafter product solids from the run, with an averageparticle size of 300 to 400 microns, was used. The recycled solids werefed via four l-inch diameter tubes evenly spaced about the outside ofthe centrifugal atomizer at a rate of 5 12 pounds per hour (recycleweight ratio of 4).

Hot air was introduced into-the dryer at about 1,000

cubic feet per minute and a temperature of 336 F. The temperature of theoutlet air was 161 F. and the temperature of the product exiting fromthe dryer was 138 F.

While operating under the foregoing conditions a rapid build-up ofaccumulated solids on the dryer walls was observed. Within an hour thetotal wall accumulation amounted to 2 19 pounds and the accumulatedmaterial had built up to a depth as high as 10 inches at some locationson the dryer walls. There was a large 7 through a centrally mass offlaked off solids over the dryer solids product outlet. Conditions werejudged to be so poor that the run could no longer be continued.

The following example illustrates the practice of the present invention.

EXAMPLE 1 A subsequent run was conducted in the same equipment and undersimilar conditions as those described in comparative Example A. In thisrun the syrup feed rate was equivalent to 147.5 pounds of syrup solidsper hour via a centrally located single fluid spray nozzle and therecycle solids were fed at a rate of 390 pounds per hour (recycle weightratio of about 2.65). The recycled solids were made up from the productof a previous run for start-up and thereafter from product of the run inprogress.' All recycle product solids were deagglomerated to an averageparticle size within the range from 50 to microns prior to being fed tothe dryer by pneumatically conveying them through a circuitous recycleconduit and introducing them to the dryer located centrifugal powderatomizer located above the syrup atomizer. The inlet air temperature was445 F. and the outlet air temperature was 194 F.

The recovered product was highly agglomerated microspheres havingagglomerate particle size ranging up to 1 [32 inch to one-sixteenthinch.

During the course of this run the dryer walls remained very clean, withonly minor wall accumulations. No difficulty was experienced with anylarge solids masses plugging the outlet. Total wall accumulation afteralmost 3 hours operation was only about 15 pounds. The general conditionof the dryer was judged to be quite satisfactory for virtually unlimitedcontinuous operation. Furthermore, all product recovered from the dryerwas dry and free-flowing and could be directly recycled withoutrequiring any storage or other further post-crystallization treatment.

What is claimed is:

1. In a process for continuously preparing dried, solidsucrose-containing product from a sucrose-containing solution bycontinuously drying droplets of the said solution in a current of heatedair and in the presence of 0.5 to 4 parts by weight of separately andcontinuously introduced recycle product solidsper part by weight solidsin the solution to be dried, the improvement which comprises inhibitingor preventing accutreatment by continuously maintaining the averageparticle size of the recycle solids at about 200 microns or less anddistributing the said separately introduced recycled product solids atthe outerperiphery of the drying zone as an enveloping solids-bearingatmosphere about the solution droplets.

2. Improvement as defined in claim 1 in which the size of the recyclesolids is maintained within the range of about 50 to about 150 microns.

3. Improvement as defined in claim 2 in which the size of the recyclesolids is maintained within the desired range by deagglomeration of theproduct in the dryer recycle system.-

4. Improvement as defined in claim 2 in which the said recycled productsolids are tangentially fed through plural'distribution inlets at theouter periphery of the drying zone.

5. Continuous-process for preparing dry sucrose concontaining solutioncontaining from about 0.25 to about 2 parts by weight of solids per partby weight of said particles;

c. evaporating the water from the solution droplets;

d. continuously separating the resulting dry crystalline, free-flowingproduct from the hot air stream;

e. reducing the average particle size of at least the necessary amountsof said resulting dry product from step (d) to provide the particlesused in step (a) to about 200 microns or less;

f. continuously recycling the necessary amounts of reduced size materialfrom step (e) to the dispersion step (a); and

g. recovering, without any post dryer crystallization treatment, theremainder of the dry product.

6. Process as defined in claim 5 wherein the size reduction in step (e)is accomplished by deagglomeration of the dry product from step (d) inthe course of the recycle step (f).

7. Process as defined in Claim 5 whereinthe size reduction in step (e)is sufficient to provide an average particle size within the range offrom about 50 to about microns.

8. Process as defined in Claim 7 wherein the sucrosecontaining solutionis an affination syrup.

9. Process of Claim 7 wherein the sucrose-containing solution is alast-strike liquor from conventional sugar refinery crystallization.

10. Process as defined in claim 7 wherein the solids are dispersed instep (a) by tangentially feeding through plural, spaced distributioninlets at the outer periphery of the drying zone.

2. Improvement as defined in claim 1 in which the size of the recyclesolids is maintained within the range of about 50 to about 150 microns.3. Improvement as defined in claim 2 in which the size of the recyclesolids is maintained within the desired range by deagglomeration of theproduct in the dryer recycle system.
 4. Improvement as defined in claim2 in which the said recycled product solids are tangentially fed throughplural distribution inlets at the outer periphery of the drying zone. 5.Continuous process for preparing dry sucrose containing productscomprising: a. continuously dispersing in a current of heated air and atthe outer periphery of a drying zone as an enveloping solids-bearingatmosphere sucrose particles having an average particle size of about200 microns or less; b. continuously and separately dispersing in theheated air, and within the said enveloping solids-bearing atmosphere,spray droplets of a sucrose-containing solution containing from about0.25 to about 2 parts by weight of solids per part by weight of saidparticles; c. evaporating the water from the solution droplets; d.continuously separating the resulting dry crystalline, free-flowingproduct from the hot air stream; e. reducing the average particle sizeof at least the necesSary amounts of said resulting dry product fromstep (d) to provide the particles used in step (a) to about 200 micronsor less; f. continuously recycling the necessary amounts of reduced sizematerial from step (e) to the dispersion step (a); and g. recovering,without any post dryer crystallization treatment, the remainder of thedry product.
 6. Process as defined in claim 5 wherein the size reductionin step (e) is accomplished by deagglomeration of the dry product fromstep (d) in the course of the recycle step (f).
 7. Process as defined inClaim 5 wherein the size reduction in step (e) is sufficient to providean average particle size within the range of from about 50 to about 150microns.
 8. Process as defined in Claim 7 wherein the sucrose-containingsolution is an affination syrup.
 9. Process of Claim 7 wherein thesucrose-containing solution is a last-strike liquor from conventionalsugar refinery crystallization.
 10. Process as defined in claim 7wherein the solids are dispersed in step (a) by tangentially feedingthrough plural, spaced distribution inlets at the outer periphery of thedrying zone.